1. Field of the Invention
This invention relates to manufacturing an insulated conductor and the article produced thereby, and, more particularly, to methods of applying a plastic covering to an electrical conductor to provide a plastic covered conductor having a controlled adhesion of the plastic covering to the conductor and the article produced thereby.
2. Prior Art and Technical Consideration
Insulated electrical conductors, such as those employed in telephone installations, are often subjected to outdoor use or to conditions that expose the insulation to the deteriorating influences of light, weather, and abrasion.
For example, it is important that plastic covered drop wire which brings telephone service from a telephone pole to a user be covered with an insulation material which has adequate properties to withstand exposure to the elements, as well as adequate low temperature flexibility, impact resistance, and abrasion resistance. An insulated conductor referred to as bridle wire which is used to field connect ends of open circuits and which is used in high density metropolitan areas must possess these same aforementioned qualities. Bridle wire may include a single conductor or a twisted pair.
The adhesion of the insulation to the conductive element in these wire products must fall within a predetermined range. Insufficient adhesion of the insulation to the conductors could cause the insulation to pull from the conductors with the entire weight of the wire in a catenary or in a vertical run held undesirably by the terminal connection. Insufficient adhesion could also lead to insulation cracking under wind loading which undesirably could allow moisture to contact the conductive element resulting in impairment of transmission quality and subsequent corrosion of the conductive element.
Too great an adhesion could require an excessive scraping activity during a stripping operation. This would remove some of the metallic material from the surface portion of the conductors thereby increasing the electrical resistance and changing the conductivity thereof, as well as reducing the strength properties of the wire to the detriment of its weight-supporting capability.
A composition of matter for producing a plastic covered drop wire was disclosed in U.S. Pat. No. 3,579,608 issued on May 18, 1971 to John B. DeCoste. See also Cogelia et al "All Vinyl Insulation for Aerial Drop Wire" pages 181-186 of Bell Telephone Laboratories Record, 6/74. One of the constituents in the DeCoste composition is a brominated epoxy resin while another uses a non-brominated epoxy resin. It has been found that the required adhesion values may be obtained by preheating a conductive element to within a predetermined temperature range and then extrusion coating one of the aforementioned compositions thereover. See copending commonly assigned application Ser. No. 412,362 filed Nov. 2, 1973 in the names of E. J. George, J. Salter, E. S. Sauer and C. E. Tidd and now U.S. Pat. No. 3,935,369, issued Jan. 27, 1976, incorporated by reference hereinto.